Adaptive predictive coding (APC) methods are widely used for high quality coding of speech signals. The details are discussed in U.S. patent application Ser. No. 07/603,104 by the present inventor and commonly assigned to COMSAT and which issued as U.S. Pat. No. 5,206,884 on Apr. 27, 1993. That application is herein incorporated by reference.
The concept of prediction filtering followed by residual quantization forms the basis for a wide range of coding techniques at various bit rates and quality for voice signals. The most direct implementation of this concept is found in adaptive predictive coding (APC) (B. S. Atal, "Predictive Coding of Speech at Low Bit Rates," IEEE Transactions on Communications, Vol. Com-30, No 4, April 1982). In APC, signal correlations are significantly reduced by adaptive short and long term prediction filters. The residual signal is then quantized by an adaptive quantizer, inside a quantization noise feedback loop. The adaptation ensures that the parameters of the predictors and the quantizer match the characteristics of the quasistationary input signal, so that the efficiency of these operations is maximized. In forward block adaptation, the signal is processed in blocks and parameters are determined for each block based on the uncoded signal. This form of adaptation requires the transmission of the prediction and quantization parameters along with the transmission of the residual. Backward sample adaptation is also possible, leading to analysis by synthesis schemes such as the low delay code excited linear prediction (LD-CELP). The proposed invention is relevant to the forward adaptive schemes.
The size of the block is highly dependent on signal characteristics and in particular on the quasistationary behavior of the signal. For telephony voice signals, sampling rates are generally in the range 6.4-8 kHz. At these sampling rates, block sizes are in the range 160-256 sample/block. For generality, block size will be denoted by N in the following discussion.